Hey everyone,
I started a thread about this MIDI sequencer I am repairing here, before i came to realize there are larger problems:
https://www.eevblog.com/forum/beginners/dead-speaker-repair-diagnosing/ For anyone wondering about the title, basically MIDI is a serial protocol sent over a 5 pin DIN, specification sends +5v on pin 4. It's common for sequencers and other "master" controllers to send 5v on that line. It is not very common input devices like keyboards, controllers etc do. I have the service manual for this device and will quote how it describes the serial I/O implementation:
5.00 MIDI I/O
The MIDI hardware is a standard implementation. MIDI out begins at the 8031's TXD port
(pin 11) and travels via R6 to the darlington pair Q1/Q2. Note that the 8031's internal pullup is not
very strong, and older units (revision A) may require the addition of an external pullup resistor for
the MIDI out to function correctly (see section 7.7).
MIDI in consists mostly of the opto isolator (U4), protection diode D6, pullup R7, and
threshold resistor R5. Note that the threshold resistor may need to be changed in order to eliminate
false MIDI triggers (see section 7.6).
After attempting to revive the speaker with no luck, i continued using it, and came to realize it was not recording and input from a MIDI keyboard properly. Only a couple notes made it through. I switched to a known good sequencer (the same model) and everything worked as expected.
Since sometimes notes would make it through and sometimes not, and i happened to know the MIDi keyboard i was using didn't send 5v on pin 4 (most don't, but it's common for sequencers to send 5v), i decided to try with a keyboard I knew sent 5v on pin 4. Suddenly everything worked almost perfectly - once in a while the sequencer would mix up a note and accidentally play it back the note next to it (for instance, if I press key 50, it might register key 51 once in a while).
I then took out the PCB to look for damage, and saw none, but i noted the MIDI DIN connectors were ever so slightly loose, so i added new solder to all of them. I noticed the reliability of recognizing incoming notes improved just slightly.
I explained my situation to a friend who then suggested I replace the optoisolator. Does anyone else have suggestions for what i may try? The circuit makes use of a couple 2N4401 transistors, so i may attempt to replace them with new 2N4401 i have here. I don't have any resistors to replace but i will order some when I buy a new optoisolator.
I'm wondering if the optoisolator could have anything to do with the missing click track referenced in the other thread, or if that's indicative of a different problem.